The present invention relates to amplifiers in general, and more particularly to a monolithic wideband amplifier.
In a transistor amplifier, high frequency behavior is limited by diffusion capacitance which is intrinsic to the device, the predominant capacitance being that of the base-emitter junction. The small-signal current gain (h.sub.fe or .beta.) as plotted versus frequency remains fairly flat over a range of low frequencies, and then rolls off at about 6 decibels per octave as the frequency is increased beyond the beta cutoff frequency f.sub..beta., the frequency where the magnitude of current gain .beta. is 3 decibels down from its low frequency value. The frequency at which the current gain decreases to unity is designated f.sub.T.
Darlington composite transistors are useful as input amplifiers because they provide a high input impedance and they are operable at low biasing currents. The small-signal current gain at low frequencies is equal to .beta..sub.1 .beta..sub.2, or simply .beta..sup.2 where the two base-emitter junctions of the Darlington pair are substantially identical. In plotting the frequency response, the logarithmic value of gain rolls off at a rate of 12 decibels per octave as frequency is increased above the .beta.-cuttoff frequency point. In general, 9 or 10 decibels per octave is considered to be the maximum allowable for good amplifier stability.
Associated with the transfer function of an amplifier and the frequency response thereof is the phase response of beta. For a single transistor amplifier, the phase shift ideally is zero at about 0.1f.sub..beta., 45 degrees at f.sub..beta., and 90 degrees at about 10f.sub..beta.. For a Darlington-connected pair of transistors, the maximum phase shift over the frequency range is near 180 degrees because two base-emitter junctions are involved. This makes the design of wideband feedback amplifier circuits employing the Darlington configuration difficult or impossible because the well known Bode criteria for a stable condition must be satisfied to prevent instability or oscillation.